Recovery of chlorine from magnesium chloride electrolysis



April 23, 1946 A. P. JuLlEN Erm. 2,398,891

RECOVERY OF CHLORINE FOM MAGNESIUM CHLORIDE ELECTROLYSIS Filed oct. ze, 1942 ATTGRNEY Y with hydrochloric aci`d or sulfuric acid,

Patented Apr. A23, 1946 RECOVERY fOl' v CHLORINEk 'FROM MAGNE` SIUM C HLORIDE ELECTROLYSIS Arne P. Julien and James x. Fai-reu, syracuse,

N. Y., assignors to The Solvay Process pany, New York, N. York Com- Y., a corporation of New 'Application october 2 6, 1942, seriamauaasss (ci. caf-219) Claims.

This invention relates-to improvements in the electrolytic process for producing magnesium metal fromv magnesium chloride, which improvements permit the recoverylof chlorine from th'e dilute4 chlorine gases evolved during the electrolysis.

As is well known, both magnesium metal and chlorine are in great demand at the present time; as a result it has become increasingly important to recover chlorine from dilute gaseous mixtures containing it. Magnesium may be economically obtained by electrolyzing magnesium chloride; during the electrolysis a dilute chlorine mixture, containing, for example, from 3% to 6% chlorine by volume and varying amounts of hydrogen chloride, is evolved. However, the recovery of chlorine from this mixture has presented mani- 4 If hydrogen chloride and chlorine 'are present in the gaseous mixture to be processed in apy about 0 and about 30 C. in order to achieve maximum absorption of the hydrogen chloride.

proximately equimolecular amounts, as is frequently the case with the dilute chlorine-hydrogen chloride mixturev recovered from the electrolysis of magnesium chloride, the 4amount of hydrochloric acid produced will be substantiallyequal to the amount required to liberate chlorine Y as hereinafter described, so that the acid thus fold difculties so that the mixture has either been discarded to waste or the valuable chlorine product converted to hydrogen chloride; accordingly, the economic values inthe dilute chlorine mixture evolved during this electrolysis have not Y been fully utilized. 4

It is an object of this invention to provide a process i'or the recovery of chlorinefrom dilute gaseous chlorine-hydrogen chloride mixtures prodiuced during the electrolysis oi magnesium chlor e.

In accordance with this invention, chlorine is recovered from a dilute gaseous chlorine-hydrogen chloride mixture produced during the electrolysis of magnesium chloride by absorbing the hydrogen chloride in water, then contacting the residual gas with calcium or magnesium carbonate" and water, preferably under conditions such that a reaction product comprising an aque'- ous solution of calcium or magnesium chloride and hypochlorous acid containing not morethan 30 grams of hypochlorous acid per liter is pro-v duced, and treating the solution thus obtained whereby a substantially undiluted chlorine gas is evolved which may be used or recovered as desiredf Our invention is economical to operate and permits the `recovery of substantially al1 of th'e chlorine gas present in the mixture evolved from the cell in substantially undiluted form, thus enhancing the economy o1' the over-allmagnesium chloride electrolytic process as well as providingi additional supplies of chlorine.

In carrying out the process of our invention the dilute'chlorine-hydrogen chloride mixture. lwhich' may contain from about 3% to about 6%l chlorine by volume and similar amounts of hydrogen chloride, is rst contacted with water to absorb the v hydrogen chloride in the form of hydrochloric acid. Contact of the mixture with the water is preferably eiected at a temperature between produced may be advantageously employed for this purpose. If desired, however, additional hydrogen chloride may be incorporated in the system, either by direct addition o f this substance or by conversion oi.' an appropriate portion of the chlorine to hydrogen chloride by burning the gaseous mixture with hydrogen or a hydrocarbon gas, in `order to permit the production of an amount of hydrochloric acid equal to that required to liberate the chlorine from the hypO- chlorous acid, produced as hereinafter described.

The dilute chlorine mixture which passes from the hydrogen chloride absorbing unit is then con# tacted with water and calcium or magnesium carbonate, preferably calcium carbonate, to convert the chlorine contained therein to calcium chloride and hypochlorous acid. The carbonate may be disposed in the form of lumps within a tower having an inlet for the chlorine mixture,

at the base thereof and a water inlet near the top so that the gas contacts the carbonate lumps while rising through the tower countercurrently to the ow of water; preferably, however, an

aqueous suspension of nely divided calcium or magnesium carbonate is passed downwardly through a tower provided with suitable packingwater, or of chlorine gas and aqueous carbonate suspension, through the reaction zone should be so adjusted that the resulting chloride-hypochlorous acid solution contains not more thanabout 30 grams per liter of yhypochlorous acid, since we have found that ii the hypochlorous acid content of the solution produced is permitted to greatly exceed this value, appreciable losses of this acid, from which the chlorine is subsequently recovered, are incurred. The temperature at which this reaction is carried about 0 C. and'about 35 C.

The aqueous chloride-hypochlorous acid solution produced as above described is then treated out may be between-V with hydrochloric or sulfuric acid in order to evolve a substantially undiluted chlorine gas. ,If the solution is treated with hydrochloric acid, the

hydrochloric acid andl hypochlorous acid interact to form chlorine and water, a substantially undiluted chlorine gas being thus evolved; the calcium or magnesium chloride remains unchanged. Treatment of the solution with sulfuric acid causes the sulfuric acid to react with the chloride out may be between about and about 50 C.

when using hydrochloric acid and between about vland about 75 C.'when using sulfuric acid. lt is advantageous to employ reduced pressures such .as about 1A; atmospherein order to accelerate the reaction by facilitating removal of chlorine gas from the reaction zone. The ,chlorine recovered may be used as such or dried by contact with sulfuric acid and then liqueed.

The calcium chloride 'solution remaining after liberation of the chlorine may be recycled through the zone in which reaction of the chlorine and carbonate occurs tobuild up the calcium chloride concentration to a point such that the calcium chloride may M economically used or recovered therefrom. When operating in this er we have found it is important to avoid building up the calcium chloride concentration in the reaction zone above about 110 grams per liter, because higher concentrations of the chloride adversely aect the rate of reaction between the calcium carbonate and chlorine, resulting in' less economical operation.

v.d speciilc embodiment of our invention is illus- Atrated diaatically in the drawing. As shown in the figure magnesium chloride is electrolyzed in electrolytic cell i, from which is withdrawn magnesium metal and a dilute chlorinehydrogen chloride mixture, the chlorine and hydrogen chloride contents of which are substantially equal and are between about 3% and about 6% by volume; the balance of the mixture is principally air and water vapor. This mixture ed to HCl absorber 2 wherein it is conwith water at 20 C. in order to absorb the hydrogen chloride from the mixture and' thereby form hydrochloric acid of about 15% concentration. The chlorine-air mixture .from absorber 2 and is introduced into the base of reactor t at the rate of about 140 cubic feet per minute. Reactor 8 is'preferably a packed acid reacts with the hypochlorous acid to form chlorine which is withdrawn to a recovery zone (not shown). In the yinitial stages of the procedure the calcium chloride solution withdrawn from desorber d is returned to reactor 3 in order to build up the calcium chloride concentration in the solution withdrawn from desorber d to a point such that it may. be economically used to react with magnesium oxide as hereinafter described; for this purpose the calcium chloride solution withdrawn from desorber li should preferably contain between about 90 and 100 grams of the chloride per liter. When the desired calciur'n chloride concentration has been attained, a portion of the solution only is returned to reactor Si in order to maintain the desired concentration and the remainder is passed to reactor t.

In reactor 5 the calcium chloride is reacted with carbon dioxide and magnesium oxide, which may be in the form of calcined dolomite or crude brucite, to yproduce calcium cartmnatc,l which is returned to reactor 3, and a solution of magnesium chloride which is withdrawn to evaporator 6; in evaporator 6 water is removed, and

the recovered magnesium chloride, usually containing water of hydration, then passed to electrolytic cell i. Make-up magnesium chloride may, if desired, be introduced into evaporator ii.

In another embodiment of our invention, particularly applicable in locations where a supply of calcium chloride is available, forexample in the form of waste liquors from ammonia-soda operations, the calcium chloride may be reacted with carbon dioxide and brucite or calcined dolo- -mite to form calcium carbonate and magnesium in connection with the specific embodiment illustower, through which tower an aqueous suspension containing about 7.0 grams per liter of iinely divided calcium carbonate is descending at the rate oi about 70 liters per minute, the temperature of the reactants in the tower being about 20? C. In reactor 3 the chlorine reacts with the calcium carbonate and water to forman aqueous solution oi calcium chloride and hypochlorous trated in Figure l. However, in the embodiment under consideration, due to the availability of large amounts of calcium chloride, it is not desirable to recycle the calcium chloride produced tion and converted to magnesium chloride for subsequent electrolysis. At present'this. conversion is accomplished by reaction with hydroacid; under the conditions above speciiled'a. so-

lutioncontaining 'not more than 30 grams per liter of hypochlorous acid, e'. g. about 7.5 grams per litem is produced. Carbon dioxide generated by the reaction and air introduced with the chlorine are vented tothe atmosphere.

The calcium chloride-hypochlorous acid solution is' withdrawn i'rom reactor 3 to desorber 4 wherein it is mixed with the hydrochloric acid produced in HC1 absorber 2. The hydrochloric chloric acid obtained by burning the chlorine in the gases evolved from the cells with hydrogen. In accordance with our invention, howeven'a portion or all of the magnesium hydroxide is -converted to magnesium chloride by reaction hydrogen chloride gas evolved from the cells may then be treated as described in connection with I reaction with the chlorine may advantageously be produced by reacting calcium chloride, produced, for example, during the chlorine-carbonate reaction, with a portion of the available magnesium hydroxide and with carbon dioxide,

whereby the desired carbonate is obtained insubstance, or mixtures of .this substance with hydrogen chloride, however produced, since the chlorine may be subjected to the reactions hereinabove described in substantially the same manner in order to recover concentrated chlorine gas.

Thus, gases containing as little as 2.5% or less chlorine or as much as 50% chlorine may economically be processed. However, because of the well integrated procedures which our invention 2. A process for the recovery of chlorine from a gaseous mixture containing this substance which comprises contacting the mixture With water and a carbonate selected-from the group,

consisting of calcium and magnesium carbon'- ates, correlating the `rates of flow of the aqueous and gaseous phases so that a reaction'product is produced comprising an aqueous solution of a chloride selected from the group consisting of calcium and magnesium chlorides and hypochlorous acid in a concentration of not more than grams of hypochlorous acid per liter, and treating the solution thus obtained with an acid selected from the group consisting of hydrochloric and sulfuric acids. y

3. A process for the recovery of chlorine from a dilute gaseous chlorine mixture which comprises contacting the mixture with Water and calcium carbonate, correlating the rates of ilow of the aqueous andgaseous phases so that an aqueous calcium* chloride-hypochlorous acid solution containing not more than 30 grains of hyprovides when applied to the recovery of chlorine f from dilute gases produced in the electrolysis of magnesium chloride, our invention will ilnd preeminent use in this latter field.

It will be evident from the above description that the process of our invention provides a simple and economical method for the recovery of chlorine from dilute chlorine mixtures, thus making possible the recovery of this valuable `material from a great many mixtures which heretofore were passed to waste, as well as permitting the economical employment of processes for the manufacture of chlorine which up tothe present time were uneconomical because dilute chlorine gases were obtained therefrom. Accordingly, the process oi our invention will be of interest to those engaged inthe manufacture of this valuable material.

Since certain changes may be made in carrying `out the above process without departing being between about 0 andabout 35 C., correlating the rates of ilow of carbonate suspension pochlorous acid per liter is produced, and treating the solution thus formed with hydrochloric acid to evolve a substantially undiluted chlorine pension of ilnely divided calcium c'arbonate is descending, the temperature within the tower and chlorine mixturethrough the tower sol that lan aqueous calcium chloride-hypochlorous acid solution containing not more than 30 grams of hypochlorous acid per liter is produced, treating the solution thus formed with hydrochloric acid at a temperature between about 20 and about 50C., and recovering a substantially undiluted chlorine gas.

5. A process for the recovery of chlorine from a dilute gaseous chlorine-hydrogen chloride mixture which comprises absorbing the hydrogen chloride in water to form hydrochloric acid, introducing the remaining gas into the base of a packed tower through which an aqueous suspeny sion of nely divided calcium carbonate is descending, the temperature within the tower being between about 0 and about 35? C., correlating the rates of iiow of carbonate suspension and chlorine mixture through-the tower so that an aqueous calcium chloride-hypochlorous scid soi lution containing` not more than 30 grams of hypochlorous acid `per liter is'prcduced, treating chloride with carbon dioxide and a substance selected from the group consisting of brucite and calcined dolomite, electrolyzing thel magnesium chloride thus produced, withdrawing the dilute chlorine-hydrogen chloride mixture generated from the cell, absorbing Vthe hydrogen chloridel in water to form hydrochloric acid, contacting the residual gas with water and the calcium carbonate produced concurrently with the magnesium chloride used in the electrolysis, correlating the rates of flow of the aqueous and gaseous phases so that an aqueous calcium chloridehypochlorous acid solution containing ,not more than 30 grams of hypochlorous acid per liter is the solution thus formedA with hydrochloric acid at a temperature between about 20 and about 50 C., and recovering a substantially undiluted chlorinegas. .n

6. A process for the recovery` of chlorine from a dilute gaseous chlorine-hydrogen chloride mixture produced in the electrolysis of magnesium chloride, which comprises contacting the gaseous mixture with water and calcium carbonate, correlating the rates of -flow of the aqueous and gaseous phases sc that an aqueous calcium chicride-hypochlorous acid solution containing not more than 30 grams of hypochlorous acid per liter is produced, .and treating the solution thus formed with hydrochloric acid to evolve a sub- Annualssunniti. .munsxrsaamn 

